Discord: channel list and presence without the main bot (server-26#225 S5)

A server linked to an org now shows in the node picker while a radio bot the
org may use is in it, even when the DRB server bot is absent or left. Its
voice-channel list is read from that radio bot's cached REST snapshot
(discord_rest.guild_directory, the same snapshot the permission check reads,
so no extra Discord request): inline when no list is stored, in the
background when the list is older than discord_directory_refresh_seconds.
The server bot's own fresh list still wins (back-compat) and its directory
push now drops the server's cached snapshots.

Guards: a server acting only through the unlinked-server fallback org still
needs the server bot; REST never creates a discord_guilds doc, never writes
org_id or bot_present, and never writes onto a server unlinked meanwhile.

Also fixes server-26#251: an interactive start (/join, admin page, web
picker, node discord_join) re-reads a cached "denied" snapshot older than
10 s once before refusing, so "fix it in Discord, then try again" works on
the next try. The reconciler never re-reads (no REST loop).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/app/routers/discord.py

@@ -138,6 +138,7 @@ async def start_stream(body: StreamStart, _: dict = Depends(require_service_key_
             mode=body.mode, channel_name=body.channel_name, guild_name=body.guild_name,
             created_by=body.created_by, preferred_token_id=body.preferred_token_id,
             max_streams=settings.discord_guild_max_streams, stream_id=body.stream_id,
+            interactive=True,   # /join, the admin page: re-read a stale "denied" (server-26#251)
         )
     except streams.StreamError as e:
         return _error(e)